Brushless permanent magnet motors have a back-EMF that is proportional to their speed. At high speeds, the back-EMF of the motor can be much higher than the power supply can deliver. Above this speed, additional current out of phase with the back-EMF must be added in order to weaken the magnetic field of the motor. This is known as “field weakening”, and is described in U.S. Pat. No. 5,677,605 assigned to Unique Mobility, Inc. This current creates electrical power losses and heat, and requires the electronics to have an increased current capacity.
One attempt to solve this problem is described in U.S. Pat. No. 6,998,757 wherein a multi-rotor synchronous machine includes first and second rotors disposed on the outer and inner periphery of a stator core. A mechanism installed inside the second rotor controls relative rotation of the first and second rotors. An electromagnetic machine with two rotors is described in U.S. Pat. No. 4,739,201. The rotors are angularly displaced relative to each other in order to reduce torque ripple, but no mechanism is described for controlling or varying the relative angular displacement between the rotors. Another electromagnetic machine with two rotors is described in U.S. Pat. No. 6,975,055, where the two rotors with field magnets are screwed onto a threaded rod.
However, none of these machines appears to have a mechanism for adjusting the rotor relative angular displacement which is simple, inexpensive, capable of operating while the machine is running and which can carry high torque loads. Furthermore, none of these machines use rotors which provide a back EMF with the rotors aligned at 180 degrees to each other.